1. Field
The presently disclosed subject matter relates to a surface light source device and an LCD unit including the same, and more particularly to a surface light source device of an edge light type having a local dimming function. Thus, the surface light source device can be employed as a light source for a back light unit located adjacent a liquid crystal display (LCD) for a television, a personal computer, mobile device, and the like, in which a light intensity of the surface light source device can vary in each local domain of light-emitting surfaces so as to match to respective regions of the LCD.
2. Description of the Related Art
Both a direct light type and an edge light type of surface light source device (when classified by location of light sources used for the surface light source devices) are known to be generally provided as surface light source devices used for a back light unit of an LCD and the like. The surface light source device of the direct light type is composed of a diffusing member, which is located rearward of an LCD, and a light source such as a plurality of light emitting diodes (LED), which is located rearward of the diffusing member so as to emit a diffusing light from the light source as a surface light source via the diffusing member. Accordingly, the direct light type device may illuminate the LCD with high brightness by increasing a light intensity of the light source, for example, by increasing the number of the LEDs.
In this case, for purpose of reducing power consumption of the light source and improving a visual quality such as a contrast of the LCD, the direct light type may provide a local dimming function, in which a light-emitting surface of the surface light source is divided into a plurality of local domains and each light intensity of the local domains is controlled by controlling the light intensity of each of the parts of the light source corresponding to each of the local domains.
When the direct light type employs a plurality of LEDs as the light source, the local dimming function may be easy to be structured by controlling each of the light intensities of the parts of the LEDs corresponding to each of the local domains. However, because a number of LEDs is required to structure the local dimming function in the surface light source device of the direct light type, the surface light source may become a high-cost device. In addition, it may be difficult for the direct light type device to allow the surface light source device to be made thinner because the direct light type needs any level of space between the LCD and the LEDs.
On the other hand, the edge light type is composed of a light guide plate, which is located rearward of an LCD, and at least one light source, which is located at least one edge of the light guide plate so as to emit light from one surface of the light guide plate toward the LCD. Thus, because an LCD unit using an edge light type device is thinner as compared to an LCD unit using the direct light type, various surface light source devices of the edge light type having a local dimming function have been devised.
For example, a first conventional surface light source device having a local dimming function is disclosed in Patent Document No. 1 (Japanese Patent Application Laid Open 2010-97909). FIG. 15a is a schematic top view depicting a basic structure for the first conventional surface light source device, which is disclosed in Patent Document No. 1, and FIG. 15b is a cross-sectional view of the basic structure of FIG. 15a taken along line A-A.
The first conventional surface light source device includes a light guide 31 including a plurality of light guide plates 31(1, 1), 31(2, 1) . . . 31(5, 1), 31(1, 2) . . . 31(1, 6) . . . 31(5, 6) arranged in a matrix as shown in FIG. 15a. Each thickness of a first light guide plate 31(1, 1) . . . 31(5, 1) arranged in rows is thinner than that of a second light guide plate 31 (1, 2) . . . 31(5, 2), which is located adjacent the first light guide plate, and the second light guide plate is arranged in a thinner shape than a thickness of a third light guide plate 31(1, 3) . . . 31(5, 3), which is located adjacent the second light guide plates as shown in FIG. 15b. 
Each thickness of the third light guide plate and a fourth light guide plate 31(1, 4) . . . 31(5, 4) located adjacent the third light guide plate is substantially the same, and the thickness of the fourth light guide plate is thicker than that of a fifth light guide plate 31(1, 5) . . . 31(5, 5), which is formed in the same thickness as the second light guide plate. Additionally, the fifth light guide plate is arranged in a thicker shape than a thickness of a sixth light guide plate 31(1, 6) . . . 31(5, 6), which is formed in the same thickness as the first light guide plate.
The first conventional surface light source device also includes a plurality of linear LED light sources 30a to 30f, and each of the linear LED light sources 30a to 30f is located at a respective one of the edges of the first light guide plate to the six light guide plate. Therefore, by controlling each light intensity of LEDs located adjacent each edge of the light guide plates 31(1, 1) to 31(5, 6) in the linear LED light sources 30a to 30f facing the respective one of the edges of the first light guide plate to the six light guide plate, the first conventional surface light source device may provide a local dimming function on each light-emitting surface of the light guide plates 31(1, 1) to 31(5, 6).
However, the first conventional device having the local dimming function may become obviously thicker in thickness as compared to a conventional edge light type, and also may be relatively more complex in structure. In addition, the number of the linear LED light source may increase as compared to a conventional edge light type. That is because the light guide 31 needs to be arranged in matrix while the first to the sixth light guide plates having a different thickness are arranged in rows, and because each of the plurality of linear LED light sources needs to be located so as to face the respective one of the edges of the light guide plates arranged in matrix.
A second conventional surface light source device having a local dimming function, in which a plurality of linear LED light sources is aligned with edges of light guides, is disclosed in Patent Document No. 2 (Japanese Patent Application Laid Open 2009-193892). FIG. 16 is a schematic cross-sectional view depicting a basic structure for the second conventional surface light source device, which is disclosed in Patent Document No. 2.
The second conventional surface light source device includes: a casing 44; a first light guide 41 including a first edge surface, a second edge surface 43a, a third edge surface 43b and a fourth edge surface 43c, and being provided in the casing 44 so as to expose a light-emitting surface (a top surface of the first light guide 41) thereof from the casing 44; and a plurality of linear LED light sources having a first, a second, a third and a fourth LED light sources 40a to 40d, the first LED light source 40a located between the first edge surface of the first light guide 41 and the casing 44, the second LED light source 40b located between the second edge surface 43a of the first light guide 41 and the casing 44 via a second light guide 42a, the third LED light source 40c located between the third edge surface 43b of the first light guide 41 and the casing 44 via a third light guide 42b, and the fourth LED light source 40d located between the fourth edge surface 43c of the first light guide 41 and the casing 44 via a fourth light guide 42c. 
In this case, each light emitted from the first light source 40a to the fourth light source 40d may enter into the first light guide 41 directly from the first edge surface or indirectly from a respective one of the second, the third and the fourth edge surfaces 43a-43c of the first light guide 41 via a respective one of the second, the third and the fourth light guides 42a to 42c. Additionally, each light intensity of the light emitted from the first light source 40a to the fourth light source 40d may be controlled by changing a driving current for each of the LED light sources 40a to 40d 
Therefore, the second conventional surface light source device may provide a local dimming function on each local domain of the light-emitting surface, which is divided between the first edge surface and the second edge surface, between the second edge surface and the third edge surface, between the third edge surface and the fourth edge surface and between the fourth edge surface and another edge surface opposite the first edge surface of the light guide, by controlling each light intensity of the LED light sources 40a to 40d facing the respective one of the first edge surface to the fourth edge surface of the first light guide 41.
However, although a structure of the second conventional surface light source device may be deferent from that of the first conventional surface light source device, principles of the first conventional and the second conventional devices may be basically similar, especially with respect to a light guide having different thicknesses and a plurality of light incident edge surfaces and a plurality of linear LED light sources corresponding to the light incident edge surfaces.
Thus, the second conventional surface light source having a local dimming function may also include the technical propositions, which are similar to these of the first conventional surface light source set forth above, such as the device may become thicker and may become more complex in structure as compared to a conventional surface light source of the edge light type.
The above-referenced Patent Documents are listed below and are hereby incorporated with their English abstract in their entireties.    1. Patent Document No. 1: Japanese Patent Application Laid Open JP2010-97909    2. Patent Document No. 2: Japanese Patent Application Laid Open JP2009-193892
The disclosed subject matter has been devised to consider the above and other problems and characteristics. Thus, embodiments of the disclosed subject matter can include a surface light source device of an edge light type having a local dimming function with a simple structure. In addition, the surface light source device can be as thin as the conventional device without a local dimming function while it provides a favorable light distribution. The disclosed subject matter can also include an LCD unit using the surface light source device as described above. The LCD unit can be configured to improve the visual quality of the display and to reduce power consumption.